This invention relates generally to a process and apparatus for forming crimped electric conductive metal foils. More specifically, the invention relates to a process and apparatus whereby such foil ribbons after pre-crimping may be compressed lengthwise to provide a high density of deep, uniform crimps per unit length and thereby increase the unit length heating surface thereof.
In the past a typical method for producing crimped foil ribbons was to pass them between two meshing gears. U.S. Pat. No. 2,731,713, for example, discloses that corrugations in a strip of resistance material may be formed by passing the material between the teeth of a pair of gears. The patent further states that the shape of the corrugations may be altered by altering the shape of the gear teeth from the conventional gear tooth cross section and that the size of the teeth of the gears determine the size of the corrugations. It is further stated that the size of the gear teeth may be of any fineness down to 100 pitch or smaller. Another patent relating to the feeding of foil between meshing gears to provide corrugations therein is disclosed in U.S. Pat. No. 2,826,105 which relates to a hand held and operated device for forming the corrugations in the foil. U.S. Pat. No. 1,345,445 relates to a machine for crimping wire wherein a swinging arm or bracket bends or loops the wire while it is being held by a pair of grooved rollers. The feeding motion of the wire is arrested during the crimping operation and then operated again to bring a new section of wire into position for crimping and the process is repeated. U.S. Pat. No. 2,683,500 relates to a filter unit and the method for producing the same, wherein wire filaments or strands are double crimped by feeding the wire or strands between a first set of intermeshing crimping rolls to impart an initial crimp end of the wire filaments or strands, and this so pre-crimped wire filament is then passed between a set of crimping rolls which act to produce large crimps in the filament and in effect serves to compress the pre-crimped metal foil. In other words, crimps of a large pitch are superimposed upon the crimps of small pitch. The wire filaments so crimped are fed to a receiving trough where they're mutually intertangled and felted together to form a filter body which is then enveloped in a jacket of tubular knit mesh.
The prior art shows no recognition of the problem solved by this invention and consequently offers no solution therefor. The problem being how to provide an electric conductive metal ribbon with a very high density of crimps per unit length and an increased ratio of crimp depth-to-pitch between successive crimps in order that a shorter length of the crimped foil ribbon having an increased electric conductive surface area might be used in the construction of small compact electric heaters and the like, which could be operated directly to 120 volts with a wattage output of less than 500 watts.
Obviously, the difficulty with providing crimps in a metal foil ribbon by passing it between two meshing gears is that there is a physical limit to the surface area of crimped foil that can be achieved in a given length. The gear-type former has a limited depth of crimp for a given pitch between successive crimps. Any attempt to go beyond this limit causes backlash between gears (one tooth gouges or undercuts the side of a mating tooth). For American Standard 141/2.degree. or 20.degree. involute gear teeth, the tooth depth is 69% of the space between teeth, regardless of gear pitch. The process of this invention allows a crimp depth of at least two times the pitch between successive crimps to be achieved. Therefore, this invention makes it possible to form a heater ribbon with more surface area per unit of finished length.